If you are thinking about buying an existing website like I am, there are many factors to consider. Sitepoint, which hosts an existing sites marketplace, has put together a great set of guidelines, and so I'll refer you to that, as well as add a few of my own thoughts here. N.B. - That's all I'll provide here: thoughts. If you need advice, seek the help of a professional.
These are things you have to think about. Since websites are entirely intellectual property, most likely copyrights, possibly trademarks, and potentially patents, you have to play by the rules.
Intellectual property gets very sticky when you have community driven content, like in a wiki or a forum. What's worse, is that almost always those sites are powered by existing software licenses, which cannot be included in the sale of the site. Thankfully, some of them are open source, so you can have your own license for free. :-)
The Sitepoint marketplace guide I read, and found very useful, can be found here:
Again, if you are considering the purchase of a valuable website, you may want to consider consulting with a lawyer before you do so.